PH-responsive viscoelastic fluid formed by cleavable sodium hexadecyl phthalate monoester alone

Abstract

Viscoelastic fluids based on wormlike micelles have attracted significant attention over the past decades, but they are generally difficult to achieve by an anionic surfactant alone. Here, we have synthesized a cleavable anionic surfactant, sodium hexadecyl phthalate monoester (PAS16). Compared with common palmitic soap, PAS16 not only has a better water-solubility (lower Krafft temperature) and a stronger ability to aggregate (lower critical micelle concentration), but can also spontaneously form a worm-based viscoelastic fluid with of pH 10.20, without the need for additives. This can be mainly attributed to the insertion of benzene rings. It was observed that the presence of 2.4% hexadecyl phthalate monoester (PAS16-H) can further enhance the viscoelasticity, resulting in a maximum at pH 7.66. This viscoelastic fluid can be quickly switched between a gel-like fluid and a low-viscosity dispersion by reversibly changing the pH between alkali and acidic conditions, accompanied with a transition between PAS16 and PAS16-H. Furthermore, the gel-like fluid can be permanently transformed into a milky dispersion through irreversible hydrolysis of the ester bond at high pH. In addition, a solution of PAS16 is more tolerant to metal ions than common anionic surfactants, which is very important in practical applications.